1. Field of the Invention
This invention relates to a process for producing foam structures having an integral, substantially non-cellular outer covering and a lower density microcellular core and to the foam structures thereby obtained. More particularly, the present invention relates to the use of expandable microspheres which simultaneously function as a blowing agent and a release agent in liquid polymeric systems used to produce shaped foam articles.
2. Description of Related Art
The manufacture of articles using liquid polymeric systems is well known in the art and has had commercial applications for a number of years. Within this field, the use of techniques designed to produce foamed or cellular polymeric materials has played an increasingly important role. By altering the manufacturing conditions and polymeric system components, objects made from thermoplastic or thermoset cellular materials may be formed to exhibit a wide variety of advantageous characteristics. For example, depending on the processing parameters selected, the resultant article may be rigid and relatively inflexible or soft and pliable. However, in either case the density of the object is less than a corresponding article constructed from the same nonfoamed material. As such, cellular constructs tend to use less polymeric matrix material with a corresponding reduction in weight and, in addition, may display desirable characteristics with respect to thermal conductivity and impact absorption.
Originally rubber was the primary material used to manufacture cellular structures. Today foamable compositions may be manufactured using numerous thermoplastic materials and thermoset resins as polymer substrates. A variety of materials may be used for the fabrication of foamed structures depending on the desired properties of the finished article. Some of the more widely used materials are polyurethanes, polyvinyl chloride, polyolefins and polystyrene. Thermosetting materials useful in the production of various foamed compositions include polyisocyanurates, polyurethaneureas, polyureas, polyesters, and acrylics, phenolics, urea-formaldehyde resins and epoxy resins. This diversity allows highly specific materials with appropriate characteristics to be fabricated.
In general the production of cellular materials involves three basic steps. First, small discontinuities or cells are created in the desired polymer matrix while it is in a fluid or plastic phase. Numerous agents may be employed for generatimg these cells. Next, using any one of several techniques, the volume of these cells are expanded to produce a polymer matrix having the desired density. Finally, the matrix material is stabilized through the use of physical or chemical means, producing a lower density article which may be modified to exhibit various attributes depending on its proposed function.
Various approaches for the production of cellular or foamed polymeric materials have been developed over the years, leading to compositions with unique structural properties. In general, the resulting product may be classified based on the process selected for foaming the polymer. Traditionally, blown cellular materials have been formed by the inclusion of a volatile blowing agent in the polymeric matrix .before molding. During processing, the incorporated blowing agent expands to form cells within the matrix, which is then fixed by chemical or physical means. A variation of this process used in the production of structural foams, combines the use of a blowing agent with a modified molding or extrusion process. Structural foams may be distinguished from simple blown material in that the structural foam possesses a dense, integral skin which is formed on the surfaces adjacent to the face of the mold. A last major class of cellular materials, termed syntactic foams, are produced through the encapsulation of hollow organic or inorganic microspheres within the polymeric matrix.
The oldest processes for producing non-syntactic foams used air as a blowing agent and mechanically incorporated it into the matrix material. While numerous blowing agents have been employed in foaming processes through the years, they usually fall into one of two categories depending on their physical state when added to the liquid polymeric system. Physical blowing agents comprise volatile liquids and compressed gases that are generally added to the polymeric melt or liquid system and later forced into a low pressure gas phase by adjusting the system parameters. In contrast to the liquid and gas phase physical blowing agents, chemical blowing agents are commonly added as solid compounds and decompose at processing temperatures to generate a gas. Various nitrogen-containing chemical blowing agents with specifically engineered decomposition temperatures are commercially available and designed for use in different polymeric systems.
For uniform cellular formation, the chemical or physical blowing agent is evenly dispersed throughout the fluid polymer melt or liquid system to create a suspension. When a physical blowing agent is employed in a polymer melt process this melt is often maintained under pressure to ensure the polymer solution is supersaturated with the blowing agent. The enriched melt is then subjected to an increase in temperature or a slight decrease in pressure which forces the blowing agent to undergo a phase change and diffuse from the polymer matrix. The diffusion and accretion of the blowing agent at discrete points in the liquid polymer melt results in cellular nucleation. When a solid chemical blowing agent is employed, individual cellular nucleation sites are produced at discrete points by generating a gas on the surface of the agent during thermal decomposition. For both types of blowing agents, cellular formation and the resulting structural composition of the foamed article may be adjusted through the regulation of reaction and/or processing parameters.
Depending on the choice of blowing agent, there are several mechanisms which may be employed during the production process to generate gas and provoke cell nucleation. One of the most common methods uses the exothermic heat generated by polymerization to volatilize a low-boiling liquid injected into the polymer components. In other models the gas is actually generated as a byproduct of chemical condensation reactions involving the crosslinking of the polymer matrix. Still other thermoplastic molding processes rely on a gas injected directly into the polymer melt and expanded simply by releasing the pressure on the system. Often these different mechanisms of cell nucleation are combined to lower the unit cost and increase the efficiency of the cellular reactions.
By modifying traditional production processes, structural foam having a non-cellular skin may be produced instead of simply blown cellular material. Such foams are characterized by an integral skin surrounding a cellular core and a high strength-to-weight ratio. These structural foams are typically manufactured using injection molding, extrusion or casting, depending on the product requirements and type of polymer used. In general it is believed that the foaming polymer system builds pressure in the mold or die and the blowing agent, which is in that layer of foam which is adjacent to the cooler mold surface, condenses causing cell collapse which results in skin formation.
The most widely used processes for manufacturing structural foams involve variations of injection molding techniques. In the low pressure process, a shot of resin or thermoplastic material containing a blowing agent is forced into the mold where the expandable mixture fills the mold cavity at pressures of 100 to 600 psi. Cellular collapse on the surface of the mold produces articles with an integral skin and cellular foam core. The high pressure process is similar except the polymeric melt and blowing agent are injected into an expandable mold. As the mold cavity expands a structural foam is produced with a highly uniform surface since the skin was formed before the expansion occurred.
One of the more dynamic areas of structural foam production involves the use of liquid polyurethane systems in reaction injection molding. This process is more efficient in producing large area, thin wall and load bearing structural foam parts. In reaction injection molding (RIM), liquid components such as polyol and isocyanate components are metered into a temperature controlled mold which is partially filled. The amount the mold is filled depends on the desired density of the finished polyurethane foamed article but typically ranges from about 20% to 60%. As the reaction mixture expands to fill the cavity, it forms a component part with an integral, solid skin and a microcellular core.
Structural foam parts may also be manufactured by using conventional liquid polymeric extruders having a specially designed die. A hollow extrudate is produced by using a die with an inner, fixed torpedo located at the center of its opening. As the outer layer of the extrudate cools it forms a solid integral skin. Following the formation of this skin, the remaining extrudate expands inward toward the center of the defined opening. The Celuka process, widely used for the commercial extrusion of foamed products, is representative of this technique.
Due to several unique and beneficial characteristics, one of the most common families of blowing agents employed in the large scale production of structural foamed material are chlorofluorocarbons (CFC's). Chlorofluorocarbon liquids become gaseous at well-defined temperatures and controllable rates, producing structural foam products with a better surface, more uniform cell structure, and overall high product yield. Furthermore CFC's leave no residue to complicate post production operations. However, due to environmental considerations (ozone depletion), the production of many CFC's, including trichloromethane or CFC 11, is to be terminated over the next few years. While other CFC's with lower ozone depleting properties are being developed, most under consideration still have adverse effects on the environment and their use will likely be restrained in the future.
In addition to CFC's, other physical blowing agents have been employed to produce structural foam products. For instance, methylene chloride is also used to produce integral skin urethane foams, but it usually yields softer products with inferior properties. Moreover, methylene chloride is quite toxic and a known animal carcinogen. Accordingly, its use is increasingly restricted by state and federal regulatory agencies. Water has also been used as a blowing agent to make polyurethane foam since it reacts with polyisocyanates and liberates CO.sub.2, which acts as a cell generator. However, due to the non-condensable nature of CO.sub.2 gas, the integral skin produced is rough and generally inferior to that produced by other methods. Limited success with water blown systems has been achieved using high G forces generated by centrifugal casting, but this technique has proved unsuitable for a wide variety of products.
In contrast, the production of structural foams using non-halogenated, low boiling point hydrocarbons as blowing agents has proven to be relatively efficient. Like CFC's these hydrocarbons possess well defined phase changes and steady, controllable vaporization rates. Furthermore such hydrocarbons leave no residue and produce structural foam having a substantially uniform cell structure and smooth integral surface.
Yet, due to their volatile nature, the use of such hydrocarbons in their liquid state poses several problems related to the handling of the material as well as ensuring its uniform dissolution within the polymeric melt. In most instances the volatile hydrocarbon must be injected or mixed with the melt under high pressures to ensure that the blowing agent will not come out of solution prematurely and interfere with the shaping process. Accordingly, the use of low boiling point hydrocarbons to produce structural foam generally requires the use of complicated and expensive production apparatus designed to maintain the entire liquid polymeric system under relatively extreme conditions until the melt is allowed to assume its final configuration.
While several techniques have been developed to ameliorate these processing and storage complications, none has proved to be entirely satisfactory. Methods developed for evenly distributing volatile liquid blowing agents in polymeric systems and storing the activated melt still require that the unexpanded compositions be kept under pressure. This leads to elevated costs due to increased equipment and energy expenditures.
For instance Wu et al., U.S. Pat. No. 4,532,094 and Kuwabara et al., U.S. Pat. No. 4,822,542 describe methods for forming cellular molded structures using particles of thermoplastic resin imbibed with volatile blowing agents. The solvent imbibed polymer particulate may be used to mold a structural foam article using a low pressure injection molding procedure. Yet, after formation the solvent imbibed particles are subject to decomposition and must be maintained under special conditions to prevent the premature vaporization of the blowing agent.
Similarly, Allen et al., U.S. Pat. No. 4,874,796 and U.S. Pat. No. 5,008,298 describe a process for producing expandable vinyl aromatic (styrene) resin beads containing a volatile blowing agent. The actual impregnation of the beads is performed in a complex aqueous solution using elevated temperatures and pressures. While the solvent imbibed styrene does react to form acceptable foam products, there is no indication that the process is capable of producing structural foam having an integral skin. Further, the specificity of the reaction solution and conditions would interfere with the use of different polymeric systems. Coupled with the complexity inherent in the solvent introduction, any such protocol would make any large scale manufacturing unwieldy and expensive.
Given the drawbacks connected with the use of volatile blowing agents in manufacturing and the limitations associated with solid chemical blowing agents, the need for different techniques to form cellular materials becomes apparent. One such method involves the use of organic or inorganic microspheres distributed throughout the polymer matrix to produce a syntactic foam. As with traditional blown foams, syntactic foams may be made using thermoplastic materials or thermoset resins. The hollow microspheres may be fixed or expandable and are usually added to the liquefied polymer matrix by simple mixing at atmospheric pressures. When using expandable microspheres, foamed objects may be produced using various forms of liquid injection molding, compression molding, reactive extrusion or casting.
Hollow microspheres have been available for over thirty years and extensively used as a filler in polymer applications. These microspheres may be constructed from inorganic materials such as glass or ceramics, or organic materials such as carbon or polyvinylidene chloride. The use of these microspheres in various resin systems to reduce density is well documented and includes their incorporation in inks, paper, various fabric applications, PVC plastisols, putties, rubbers, silicones, polyurethanes and even explosives. Inorganic microspheres tend to have rigid shells and usually maintain a constant volume during manufacturing applications. In contrast, certain classes of organic microspheres may be constructed from pliant materials and fluctuate in volume during use.
Termed expandable microspheres, these organic microspheres are usually constructed from a thermoplastic material and typically enclose a volatile gas or liquid. When subjected to heat these microspheres are capable of swelling to several times their original volume. Quite often these organic spheres are expanded prior to being mixed with a liquid polymer system thereby acting as a traditional filler. In other instances they are mixed with a polymer in their unexpanded state and swell during processing. In either case the resultant foamed products tend to exhibit a rough, pitted surface which often requires post production finishing.
A standard use for microspheres as a simple filler was described by Rex in U.S. Pat. No. 4,250,136 which discloses a method for forming a composite structure using a foam core containing a high volume of microspheres. The core matrix is thermosetting resin and is molded with the outer layer of composite material under heat and pressure. Organic or inorganic hollow spheres of 10-15 .mu.m, uniformly distributed throughout the uncured resin matrix, reportedly aid in weight reduction and impact absorption thereby protecting delicate instrumentation.
In another embodiment, a thermoset resin containing expandable microspheres used to reinforce a structural member was proposed by Wycech in U.S. Pat. No. 4,995,545. The unexpanded microspheres are activated by the heat generated in the exothermic polymerization reaction. With an original diameter of approximately 5-7 .mu.m, the spheres are reported to swell to a diameter of approximately 40 .mu.m during the polymerization reaction. This resin containing the expanded microspheres was allowed to cure in a cavity and thereby reinforce the structure. According to the teachings of the reference, the microsphere filled resin was tightly bound to the structural member.
In general, whether the cellular material is produced using hollow microspheres or volatile blowing agents, most reactive polymer systems in use today tend to adhere to equipment used to shape the foamed article. Many manufacturing protocols for cellular products require that external release agents be periodically applied to the mold to permit part removal. This is a time consuming and labor intensive step which can greatly slow the pace of production. In addition, periodic tool stripping or cleaning is often required due to the continual build-up of polymer on the mold. Additionally, the residue left on the molded cellular part must be washed off the surface prior to painting or other post-finishing operation in order to obtain good adhesion.
In the case of some selected polymeric systems, compounds exist which may increase the period between release agent applications. For instance it has been reported that the use of zinc stearate (Dow Chemical), in combination with polyether polyamines, can reduce the number of external release agent applications used for reaction injection molding (RIM) processes with polyurethanes. This technology is very specific and no single compound is broadly applicable for a spectrum of polymer casting systems. For example zinc stearate is not effective for non-amine containing polyurethane materials, e.g. those containing only polyhydroxy reactive components. Thus, while the zinc stearate may be used to reduce the number of applications of external wax or soap release agent required for selected urethane systems, a great number of other polymer systems must have an external mold release applied between every operation. Further, even when zinc stearate is available a separate release agent must be reapplied every 20-30 moldings depending on the complexity of the part.
Accordingly it is an object of the present invention to provide an improved, cost efficient process for the production of structural foam articles.
In particular it is an object of the present invention to provide an improved, cost efficient process for the production of molded articles made of cellular polyurethanes with an integral, substantially non-cellular skin.
Further, it is an object of the present invention to provide a process for manufacturing structural foam without the use of ozone-depleting chlorofluorocarbons.
Additionally it is an object of the present invention to provide a process for the production of molded polymeric foam articles without the need for continuous application of an external mold release.